What is a phono pre-amp and why do I care?

There are obviously things even clever people don’t think of when dishing out advice on turntables. Like phono pre-amplifiers for example, not to mention the delicacy of ‘phono’ inputs. Salespeople worth their salt tell customers about the traps when they’re buying a turntable while others let them waltz out the door in their ignorance, very possibly at risk of thinking they’ve bought a dud.

A slow but steady stream of reader emails makes it clear to me that a good deal of turntable buyers, especially younger folk with no life experience of these things, don’t have the first idea what a phono pre-amp is, let along what it does. And then there’s the ‘phono’ input.

The very first thing to know is whether you need a phono pre-amp and, for first-time buyers, it’s likely you do. Look at what you’re about to plug the turntable into. This will be some sort of amplifier and it will either have an input at the back marked ‘phono’ or it won’t. Serious amplifiers, and a rapidly growing number of cheaper amplifiers, have them. Less serious amplifiers and all-in-one stereos usually don’t. If you have a ‘phono’ input, plug your turntable in there – you won’t need a phono pre-amp. Otherwise you will. Unless.

Unless the turntable you have just bought has a phono pre-amp built in. Some do. If that’s the case plug the turntable into a spare input, like the auxiliary or ‘aux’ input. Don’t plug it into a ‘phono’ input or you may blow your speakers up.

“Phono pre-amps are often the unsung hero of a great turntable rig,” says Leigh Fischer, product manager at Pro-ject. “This is the part where the most gain is added to your signal, it can be critical.”

A turntable gathers its signal from an extraordinarily delicate and tiny piece of equipment, a stylus that follows the contours inside the walls of a record’s groove. As the stylus moves, a magnet attached to the other end, inside the cartridge, also moves and induces current in two coils of wire in there, creating electrical signals that are fed to the amplifier. These signals are exceedingly weak, far weaker than the signals from other sources. A CD player may have an output of a couple of volts, a cartridge’s output is more like four or five millivolts; that’s 0.004 to 0.005 volts. This means these signals need a mighty whack of amplification.

If your amplifier has a ‘phono’ plug this happens automatically inside the amp. If not, you need a phono pre-amp to bring the signal up to sufficient strength to be heard properly. Plug a turntable into an ‘aux’ input and you may not hear anything even with the volume maxed. The reverse is also true – plug something that isn’t a turntable into the ‘phono’ input and the signal will be so strong it may well blow your speakers’ voice coils. Exceedingly expensive.

Good phono pre-amps start around $150 and from there, as is customary in hi fi, the sky is the limit. If you want one with variable impedance and balanced XLR outputs you can easily go into four figures. You can even get them with valves. The variable impedance feature lets you fine-tune the response of a moving coil cartridge to the type of recording you’re listening too. It’s fun, hands-on and sounds sweet.

Make sure the phono pre-amp is compatible with your cartridge. It’s likely a moving magnet (MM) cartridge but some more expensive ones are moving coil (MC) and operate differently. Some phono pre-amps operate with both.